1. Field of the Invention
The present invention relates to magnetic bubble domain memory systems.
2. Description of the Prior Art
In the field of electronic computers and other data processing devices, the performance of such systems is largely limited by the speed, capacity and reliability of the memory system. Various types of memory systems are known and have been used in the art, such as disc files, magnetic tapes, and ferrite cores. Recently, significant interest has been directed toward a different type of memory wherein data is stored in the form of magnetic "bubbles" moving in thin films of magnetic material. The bubbles are actually cylindrical magnetic domains whose polarization is opposite to that of the thin magnetic film in which they are embedded. The bubbles are stable over a consideralble range of conditions and can be moved from one point to another at high velocity. Interest in these devices, in large part, is based on the high packing density that can be achieved and the ability of the cylindrical domain to be independent of the boundary of the magnetic material in the plane in which it is formed. Such devices are described in an article by Andrew H. Bobeck and H. E. D. Scoville entitld "Magnetic Bubbles," Scientific American, June 1971, Vol. 224, pp. 88-90. This article describes several structures for manipulating and controlling transmission of magnetic bubbles along discrete paths and includes an explanantion of one form of a magnetic bubble domain memory.
Magnetic bubble domain memory systems offer significant advantages over conventional memory systems since logic, memory, counting, and switching can all be performed within a single layer of solid magnetic material. This is in contrast to conventional memory systems in which, to perform the above functions, information must move from one devie to another through interconnecting conductors and high gain amplifiers.
In order to carry out the memory functions of a bubble memory system such as logic operations, reading, writing, et cetera, it is necessary to provide an in-plane rotating field at the surface of the bubble memory chips employed in the memory system and to drive electrical current through appropriate leads on the magnetic bubble domain chip at precisely timed intervals relative to the in-plane rotating field. These currents control the magnetic bubble domains in such a way as to allow the user to write data into or read data from any desired location on the memory chip.
Typically, the cntrol signals for synchronizing the memory operations are generated by one-shot multivibrators which depend on RC discharge circuits to produce pulses of desired width and at desired intervals during each rotation of the in-plane rotating magnetic field. Various problems, however, have been encountered in utilizing multivibrators for generating these control signals.
To overcome the problems and difficulties associated with the analog control obtainable with multivibrators, Naden in U.S. Pat. No. 3,997,877, issued Dec. 14, 1976, describes a digital control system employing a read-only memory (ROM) driven by a clock-controlled counter. The counter sequentially advances through its states, providing thereby a sequence of addresses to the ROM, and the ROM contents are arranged so that each output bit develops a specified control signal, as required for the memory system.
The substantial efforts in bubble memory art have, heretofore, been directed to improving bubble chip organization and manufacturing techniques. But still, memories having a very large capacity cannot reliably be manufactured.
It is an object of this invention, therefore, to combine bubble memory chips and to form memory systems which can be constructed to any desired capacity.
It is a further object of this invention to efficiently combine a number of bubble memories with their associated controls on a single construction unit, such as a pluggable card.
It is a still further object of this invention to form a bubble memory system on a card which, with respect to the required control signals, is indistinguishable from any other serial memory system (such as a shift register).
It is an additional object of this invention to form a bubble memory system which is digitally controlled.
It is a still additional object of this invention to form a memory system which gracefully degradates with the occurrence of a low voltage source condition.